1. Field of the Invention
This invention relates to a method of forming, by the flame hydrolysis technique, high optical purity blanks from which high quality optical waveguides, lenses, prisms and the like can be made. This invention is particularly applicable to optical waveguides which must be formed from extremely pure materials.
Optical waveguides, which are the most promising medium for transmission of signals around 10.sup.15 Hz, normally consist of an optical fiber having a transparent core surrounded by transparent cladding material having a refractive index lower than that of the core.
The stringent optical requirements placed on the transmission medium to be employed in optical communication systems has negated the use of conventional glass fiber optics, since attenuation therein due to both scattering and impurity absorption is much too high. Thus, unique methods had to be developed for preparing very high purity glasses in fiber optic form. Glass preparation techniques which have shown much promise are based on the flame hydrolysis process which employs vapor phase reaction of high purity vapors. This approach to the formation of low loss optical waveguides is based on methods described in U.S. Pat. Nos. 2,272,342 and 2,326,059 issued to J. F. Hyde and M. E. Nordberg, respectively. The flame hydrolysis technique has been employed to prepare single mode waveguides and multimode waveguides of both the step-index and graded-index type. Various methods employing the flame hydrolysis technique for forming glass optical waveguide fibers are taught in U.S. Pat. Nos. 3,737,292; 3,823,995 and 3,884,550.
The usefulness of glass optical waveguides in optical transmission systems depends upon the attainment of very low loss transmission over the entire wavelength range of about 700-1100 nm. This can be achieved by reducing attenuation due to optical scattering and absorption to a level which approaches the minimum theoretically attainable level. Waveguides in which at least 80% of the scattering loss can be accounted for by intrinsic glass scattering have been made by the aforementioned flame hydrolysis technique. However, due to the presence of residual water produced by this technique, absorption losses between 700 nm and 1100 nm have been excessively large. By residual water in glass is meant that the glass contains a high level of OH, H.sub.2 and H.sub.2 O. One explanation of residual water may be found in U.S. Pat. No. 3,531,271 to W. H. Dumbaugh, Jr. The maximum attenuation in the aforementioned wavelength range that is attributable to residual water occurs at about 950 nm. The remaining portion of the attenuation at 950 nm, which is due to factors such as intrinsic material scattering, amounts to about 3 dB/km. For example, a glass optical waveguide having an attenuation less than 6 dB/km at 800 nm may have an attenuation greater than 100 dB/km at 950 nm due to the presence of water therein. To be useful in optical communication systems, optical waveguide attenuation is preferably less than 10 dB/km at the wavelength of light being propagated therein. In order to achieve such low attenuation over the entire range between 700 nm and 1100 nm, a glass waveguide fiber must be rendered substantially water-free, i.e., the amount of residual water within the fiber must be reduced to a level of less than 10 ppm.
2. Description of the Prior Art
Since it is impossible to reduce the water content to acceptable levels after flame hydrolysis-produced soot has been consolidated to form a solid glass coating, the water must be removed before or during the consolidation process. Heretofore, various methods were employed to reduce the water content in optical waveguides produced by flame hydrolysis. Such disadvantages as long processing times, equipment problems and incomplete water removal were encountered.
One prior art method that has been very effective in reducing the water content in fused silica produced by the flame hydrolysis process is disclosed in U.S. Pat. No. 3,933,454. In accordance with that patent a soot preform produced by the flame hydrolysis process is consolidated by inserting it into a consolidation furnace wherein the soot preform is heated to a temperature within the consolidation temperature range for a time sufficient to cause the soot particles to fuse and form a dense glass layer. The soot preform is simultaneously subjected to a stream of a substantially dry chlorine containing gas which flows through the furnace. The chlorine permeates the interstices of the soot preform during the consolidation thereof and replaces hydroxyl ions by chlorine ions, thereby resulting in a glass article that is substantially water-free. However, prior to making contact with the soot preform, the chlorine containing gas can react with the walls of the consolidation furnace to produce volatile compounds such as iron chlorides which can then contaminate the preform. Thus, while the resultant glass article exhibits very little excess attenuation at 950 nm due to water absorption, the overall attenuation thereof across the entire visible spectrum is increased due to impurities transported by the drying gas.